Why is the friction pair considered the most important part of a rotary joint? This is because the service life and working quality (leakage index) of the rotary joint are directly related to it. According to statistics, approximately 80% to 95% of the leakage in rotary joints is caused by the sealing end face and the friction pair.
The materials for the friction pair should meet the following conditions:
(1) High mechanical strength, capable of withstanding pressure and pressure deformation;
(2) Good dry grinding resistance, high load-bearing capacity, and good self-lubrication;
(3) Good compatibility between paired materials, with no excessive wear or galvanic corrosion;
(4) Good wear resistance and long service life;
(5) Good thermal conductivity and heat dissipation;
(6) Good high-temperature resistance;
(7) Good thermal cracking resistance;
(8) Strong corrosion resistance;
(9) Small linear expansion coefficient, good resistance to thermal deformation and dimensional stability;
(10) Good machinability and formability;
(11) Good air tightness;
(12) Low density.
The materials for the friction pair of rotary joints should be selected based on the nature of the working medium, working pressure, temperature, and rotational speed of the rotary joint. Especially when upgrading or modifying old equipment or retrofitting existing rotary joints, it is crucial to choose the appropriate sealing materials for the rotary joint.
When the rotary joint is in a corrosive medium, it is subject to both chemical and electrochemical corrosion, especially on the friction surface, where the corrosion rate is extremely high. This is because the corrosion products just formed on the end face (which can reduce the corrosion rate) are destroyed by friction. This periodic cycle of the formation and removal of the corrosion-resistant surface is commonly referred to as erosion. The erosion rate is about 10 to 50 times that of the surface without friction. Therefore, the friction pair should be made of materials that are both corrosion-resistant and wear-resistant.
Generally speaking, phenolic resin-impregnated graphite is resistant to acids but not to alkalis, furan resin-impregnated graphite is resistant to both acids and alkalis, epoxy resin-impregnated graphite has good alkali resistance, and polytetrafluoroethylene resin-impregnated graphite and ceramics can withstand strong corrosive media.
(1) When the rotary joint is in a high-pressure medium, if the end face pressure ratio caused by the high-pressure medium exceeds its applicable range, it will lead to the destruction of the liquid film, causing heat generation and wear. High pressure can easily cause deformation and fracture of the friction pair, resulting in the failure of the rotary joint seal. Materials for the friction pair that work under high pressure must have sufficient rigidity and strength. Hard alloys, ceramics, and sprayed ceramics with high pressure resistance, strength, and rigidity should be considered.
(2) In high-speed rotation conditions, due to the large linear speed of the rotary joint seal, the friction heat and wear on the sealing surface increase. Therefore, materials with low friction coefficients and good thermal conductivity, such as copper-impregnated ceramic graphite, should be used for the friction pair.
(3) When the rotary joint is in a high-temperature environment, if not properly handled, it can lead to the aging of rubber products and the deformation of parts, causing leakage in the rotary joint seal. Materials with good high-temperature resistance, such as sintered alumina ceramics (corundum ceramics) and hard alloy surfacing, should be selected.
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